Source: http://www.technologyreview.com/news/529...

 you’re going to choose a place on the body to measure physical signals, Steven LeBoeuf says two places are far and away the best: the ear or the rear.

LeBoeuf knows it sounds like a joke, but he’s not kidding. He’s the president and cofounder ofValencell, a company based in Raleigh, North Carolina, that developed and licenses technology it says can accurately track vital signs like heart rate, temperature, and respiration rate from the same earbuds you’d use to listen to music. It does this with photoplethysmography, or PPG, which measures changes in blood flow by shining a light on the skin and measuring how it scatters off blood vessels (this is often done in hospitals with a device that fits over your fingertip).